1. Field of the Invention
The present invention relates to a process for preparing aldehydes by hydroformylation catalyzed by metals of groups 8 to 10 of the Periodic Table of the Elements in the presence of cyclic carbonic esters.
2. Discussion of the Background
The reactions of olefin compounds, carbon monoxide and hydrogen in the presence of a catalyst to form the aldehydes having one more carbon atom are known as hydroformylation (oxo process). Catalysts used in these reactions are frequently compounds of the transition metals of groups 8 to 10 of the Periodic Table of the Elements, in particular compounds of rhodium and of cobalt. In comparison with catalysis by cobalt compounds, hydroformylation using rhodium compounds generally offers the advantage of higher selectivity and is therefore usually more economical. The rhodium-catalyzed hydroformylation is usually carried out using complexes comprising rhodium and preferably trivalent phosphorus compounds as ligands. For example, compounds from the classes of phosphines, phosphites and phosphonites are frequently used as ligands. An overview of the hydroformylation of olefins may be found in B. CORNILS, W. A. HERRMANN, “Applied Homogeneous Catalysis with Organometallic Compounds”, Vol. 1&2, VCH, Weinheim, New York, 1996.
Hydroformylations are frequently carried out in the presence of solvents so that the catalyst can easily be recirculated after the reaction product has been separated off. In many continuous hydroformylation processes in which rhodium catalysts are used, the high-boiling mixtures formed as by-product in the hydroformylation are employed as solvents. Such processes are described, for example, in DE 2 062 703, DE 2 715 685, DE 2 802 922 or EP 017183.
In addition to the high boilers, it is possible to use inert organic liquids (DE 3 126 265) and reaction products (aldehydes, alcohols), aliphatic and aromatic hydrocarbons, esters, ethers and water (DE 4 419 898) as solvents. In GB 1 197 902, saturated hydrocarbons, aromatics, alcohols and n-paraffins are used for this purpose.
The addition of one or more polar organic substances in the hydroformylation process is disclosed, for example, in WO 01/68248, WO 01/68249, WO 01/68252. For the present purposes, polar substances are substances from the following classes of compounds: nitrites, cyclic acetals, alcohols, pyrrolidones, lactones, formamides, sulfoxides and water.
The use of carbonic esters as polar additive in cobalt-catalyzed hydroformylation reactions is also known (U.S. Pat. No. 3,992,453). Here, the carbonic ester is not used as solvent but as promoter in the presence of organophosphine complexes. The carbonic esters are used in a molar ratio to the cobalt compound of 1:2. In accordance with the desired catalytic action, the olefin is used in a >100 fold excess over the catalyst metal and the carbonic ester.
The simultaneous use of a polar solvent and a nonpolar solvent has likewise been described in the past (WO 99/38832, WO 01/68247, WO 01/68248, WO 01/68249, WO 01/68250, WO 01/68251, WO 01/68242). The following groups of substances are mentioned as nonpolar solvents: aliphatic, alicyclic and aromatic hydrocarbons, ethers, amines, carboxylic esters, ketones, silanes, silicones and carbon dioxide.
Reasons for the use of polar or nonpolar solvents in hydroformylation reactions are the increased catalyst stability in the reaction and easier work-up of the hydroformylation product. When the catalyst is separated off from the reaction product, e.g. by distillation, deactivation of the catalyst is frequently observed. There have therefore been many attempts to replace the work-up by distillation by a milder process, e.g. by extraction.
Thus, for example, U.S. Pat. No. 6,187,962 and EP 0 922 691 describe a palladium-catalyzed hydroformylation in the presence of sulfones or polynitriles with subsequent separation of the product phase from the catalyst-containing phase and recirculation of the latter. In U.S. Pat. No. 5,648,554, a selective extraction of high boilers and a selection extraction of the catalyst complex using polar solvents such as water, ketones, alcohols, nitriles, amides, diols and carboxylic acids are carried out. U.S. Pat. No. 5,138,101 describes the extraction of the reaction product with alcohol/water mixtures.
In summary, it can be said that a large number of polar and/or nonpolar solvents have been used in hydroformylation reactions.
A person skilled in the art will know that the majority of the solvents mentioned are not at all inert under the conditions of the hydroformylation. For example, aldehydes can react with customary phosphite ligands. The addition of water and/or a carboxylic acid can lead to hydrolytic decomposition of phosphite, phosphonite and phosphinite ligands. Amides can, owing to their complexing properties, displace ligands from the metal center. Alkadienes are known as catalyst poisons. (P. W. N. M. van Leeuven in P. W. N. M. van Leeuven, C. Claver, “Rhodium Catalyzed Hydroformylation”, Kluver Academic Publishers, Dordrecht, Boston, London, 2000).
Furthermore, some of the solvents mentioned can reduce the yield by reaction with the aldehydes. Thus, for example, alcohols and diols lead to acetal formation, while the addition of carboxylic acid can catalyze the difficult-to-control aldol reaction.
Furthermore, the known hydroformylation processes are capable of improvement in terms of the selectivity to linear aldehydes, i.e. the use of an additional solvent should in the ideal case not only improve the work-up but also effect an improvement in the selectivity.
JP 10-226662 describes a process for the hydroformylation of olefinic compounds in which a rhodium catalyst is used together with a sodium salt of sulfonated triphenylphosphines as cocatalyst, i.e. a modified catalyst is used. The reaction is carried out in the presence of a polar solvent and a carboxylic acid. As polar solvent, it is possible to use, for example, dimethyl sulfoxide, sulfolane, N-methylpyrrolidone, N,N-dimethylformamide, aceto-nitrile, butanediol, polyalkylene glycols or ethylene carbonate. The polar solvent can be recirculated together with the acid and the catalyst to the hydroformylation reaction. In this process, an alkylene carbonate is used as solvent for the first time. However, a carboxylic acid has to be used in addition to this alkylene carbonate. Although this can be recirculated, the presence of this additional compound can lead to contamination of the desired target product. Firstly, contamination can be by the acid itself, or by-products can be formed by acid catalysis, e.g. by aldolization, and lead to undesirable impurities. The use of the process mentioned is additionally restricted to the hydroformylation of terminal olefins, which are comparatively reactive. In the case of less reactive olefins, viz. internal olefins and especially internal highly branched olefins, the activity of the catalyst is far below that required for industrial use.
It is therefore an object of the present invention to provide a combination of solvent or solvent mixture and ligands for use in a hydroformylation reaction, which does not suffer from the disadvantages described.